“Jumping jacks backward – let's push!” instructor Allison Lilly shouts, like a coach revving her team for a big game. “If you can say a whole sentence, you should probably work a little harder.”

The 15 women and two men bob vigorously in water 9-feet deep. Most wear float belts and webbed gloves for extra resistance.

In the two years since Lilly began teaching water fitness, she's seen more younger people signing up. Some come because they're injured or pregnant – and stay because they love the workout. Others just hear rave reviews from friends.

“It works everything,” instructor Lilly, 43, says. The resistance of the water builds muscle strength and endurance, while working both the upper and lower body.

UNC Charlotte gerontologist Rachel Seymour agrees.

But because water aerobics isn't weight-bearing exercise, which women need to guard against osteoporosis, she recommends combining it with walking or another exercise where you support your own weight.